You'd think your brain could tell the difference between a sugar pill and a dose of morphine. But researchers say they have finally confirmed a longstanding hypothesis: that placebos activate the same brain circuits as painkilling drugs.
Previously, scientists had only indirect evidence that placebos tap into the same painkilling circuits as do opioid drugs. This evidence was drawn primarily from studies showing that drugs that jam the opioid circuits also block the analgesic effect of placebos. Now, a team headed by Predrag Petrovic of Stockholm's Karolinska Institute has used brain imaging to show a more direct link. Petrovic and his colleagues used positron emission tomography to scan the brains of nine men while a 48°C metal surface was pressed to the backs of their hands. The team compared brain responses after subjects were given intravenous injections--by a doctor in a white coat--of either an opioid painkiller or a placebo.
Both the genuine painkiller and the placebo increased blood flow in areas of the brain known to be rich in opioid receptors: the brainstem and the rostral anterior cingulate cortex (ACC). Furthermore, those people who responded most to the placebo--according to their ratings on a scale of 0 to 100 of how much it reduced their pain--also showed more rostral ACC activation from the drug. This provides new fodder for the hypothesis that "high placebo responders have a more efficient opioid system," the authors write in a report published online this week by Science.
The study backs up the idea that the placebo response plays a role in all treatments for pain, says neuroscientist Fabrizio Benedetti of the University of Turin, Italy. For instance, one of his recent studies shows that a painkilling injection is more effective when the patient is watching than when the drug is administered covertly.